14 Feb 2007
Russian scientists show how bismuth-doped fibers can be used to create a simple all-fiber pulsed laser.
Bismuth-doped fibers can act as a saturable absorber (SA) and extend the lasing wavelength range of ytterbium-doped fibers out to 1200 nm, according to a trio of researchers at the Russian Academy of Sciences. Q-switched pulses produced at this wavelength are suitable for micromachining, and frequency doubling brings medical applications into play (Optics Letters 32 451).
"Bi-doped glasses exhibit broadband luminescence in the near infrared region," researcher Vladislav Dvoyrin told optics.org. "Depending on the glass composition, it could be up to 400 nm FWHM covering the entire silica transparency region from 1 to 1.7 μm. Such glasses are therefore very promising for creating broadband amplifiers for fiber telecommunication lines and tunable or femtosecond lasers."
Inserting a fiber SA into the cavity of a fiber laser is a simple way to construct an all-fiber pulsed laser. Knowing that Bi-doped fibers have a broad absorption band at 1 μm that overlaps well with the Yb luminescence band, Dvoyrin and colleagues decided to use these fibers as SAs for Q-switching Yb fiber lasers.
The researchers investigated two configurations. Their first Yb-Bi "double-cavity" laser comprised a Yb-doped phosphosilicate laser fiber fusion spliced to a Bi-doped aluminosilicate fiber using a fiber Bragg grating. A diode laser emitting at 975 nm optically pumped the Yb-doped fiber.
"At 1066 nm the Yb-Bi laser produced pulses with energies of up to 140 μJ, peak powers of up to 100 W and pulse durations could be minimized to 400 ns," said Dvoyrin. "We demonstrated frequency doubling at 1066 nm and also made a pulsed fiber Raman laser at 1256 nm pumped by a Yb-Bi laser."
In the second configuration the team spliced a piece of Bi-doped fiber to the other end of the Yb-doped fiber. This second Bi-doped fiber acted as an amplifier and allowed the team to generate Q-switched pulses.
"An additional fiber Bragg grating was spliced to the output of the second Bi-doped fiber to reflect the pump backward," said Dvoyrin. "We used the second piece of Bi-doped fiber as an amplifier because only a small part of the Yb lasing power (the pump in this case) was absorbed in the first piece of Bi-doped fiber."
According to Dvoyrin, this produced lasing between 1150 and 1200 nm. "At 1160 nm we report a pulse energy of up to 10 μJ, peak power of up to 5W, pulse duration of around 1 μs, a repetition rate of between 10 and 100 kHz and an average power of up to 500 mW with an efficiency of around 10%," he said. "We also saw a stable mode-locked Q-switched mode with duration of a single peak in a packet less than 1 ns. Frequency doubling at 1160 nm was possible."
Inspired by its initial findings, the team from the Fiber Optics Research Center now plans to increase the pulse energy up to 1 mJ, shorten the pulse duration to 100 ns and develop Bi-doped fibers capable of producing lasing action beyond 1200 nm.
"We have also licensed our Bi-doped fiber technology," revealed Dvoyrin. "Yb-Bi lasers are of great practical interest and Bi-doped fibers seem to be the best fiber SAs for Yb fiber lasers at this time."